Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace that has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products comprising olefins leave the pyrolysis furnace for further downstream processing, including quenching.
Pyrolysis involves heating the feedstock sufficiently to cause thermal decomposition of the larger molecules. The pyrolysis process, however, produces molecules that tend to combine to form high molecular weight materials known as tar. Tar is a high-boiling point, viscous, reactive material that can foul equipment under certain conditions. In general, feedstocks containing higher boiling materials tend to produce greater quantities of tar.
Conventional steam cracking systems have been effective for cracking high-quality feedstock which contain a large fraction of light volatile hydrocarbons, such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost feedstocks containing resids such as, by way of non-limiting examples, atmospheric residue, e.g., atmospheric pipestill bottoms, and crude oil. Crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points in excess of 590° C. (1100° F.). The non-volatile components of these feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatile components can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized.
In most commercial naphtha and gas oil crackers, cooling of the effluent from the cracking furnace is normally achieved using a system of transfer line heat exchangers, a primary fractionator, and a water quench tower or indirect condenser. The steam generated in transfer line exchangers can be used to drive large steam turbines which power the major compressors used elsewhere in the ethylene production unit. To obtain high energy-efficiency and power production in the steam turbines, it is necessary to superheat the steam produced in the transfer line exchangers.
Cracking heavier feeds, such as kerosenes and gas oils, produces large amounts of tar, which leads to rapid coking in the radiant section of the furnace as well as fouling in the transfer line exchangers preferred in lighter liquid cracking service.
Additionally, during transport some naphthas are contaminated with heavy crude oil containing non-volatile components. Conventional pyrolysis furnaces do not have the flexibility to process residues, crudes, or many residue or crude contaminated gas oils or naphthas which comprise non-volatile components.
To address coking problems, U.S. Pat. No. 3,617,493, which is incorporated herein by reference, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 230 and 590° C. (450 and 1100° F.). The vapors are cracked in the pyrolysis furnace into olefins and the separated liquids from the two flash tanks are removed, stripped with steam, and used as fuel.
U.S. Pat. No. 3,718,709, which is incorporated herein by reference, discloses a process to minimize coke deposition. It describes preheating of heavy feedstock inside or outside a pyrolysis furnace to vaporize about 50% of the heavy feedstock with superheated steam and the removal of the residual, separated liquid. The vaporized hydrocarbons, which contain mostly light volatile hydrocarbons, are subjected to cracking.
U.S. Pat. No. 5,190,634, which is incorporated herein by reference, discloses a process for inhibiting coke formation in a furnace by preheating the feedstock in the presence of a small, critical amount of hydrogen in the convection section. The presence of hydrogen in the convection section inhibits the polymerization reaction of the hydrocarbons thereby inhibiting coke formation.
U.S. Pat. No. 5,580,443, which is incorporated herein by reference, discloses a process wherein the feedstock is first preheated and then withdrawn from a preheater in the convection section of the pyrolysis furnace. This preheated feedstock is then mixed with a predetermined amount of steam (the dilution steam) and is then introduced into a gas-liquid separator to separate and remove a required proportion of the non-volatiles as liquid from the separator. The separated vapor from the gas-liquid separator is returned to the pyrolysis furnace for heating and cracking.
Co-pending U.S. application Ser. No. 10/188,461 filed Jul. 3, 2002, Patent Application Publication US 2004/0004022 A1, published Jan. 8, 2004, which is incorporated herein by reference, describes an advantageously controlled process to optimize the cracking of volatile hydrocarbons contained in the heavy hydrocarbon feedstocks and to reduce and avoid coking problems. It provides a method to maintain a relatively constant ratio of vapor to liquid leaving the flash by maintaining a relatively constant temperature of the stream entering the flash. More specifically, the constant temperature of the flash stream is maintained by automatically adjusting the amount of a fluid stream mixed with the heavy hydrocarbon feedstock prior to the flash. The fluid can be water.
In using a flash to separate heavy liquid hydrocarbon fractions containing resid from the lighter fractions which can be processed in the pyrolysis furnace, it is important to effect the separation so that most of the non-volatile components will be in the liquid phase. Otherwise, heavy, coke-forming non-volatile components in the vapor are carried into the furnace causing coking problems.
Increasing the cut in the flash drum, or the fraction of the hydrocarbon that vaporizes, is also extremely desirable because resid-containing liquid hydrocarbon fractions generally have a low value, often less than heavy fuel oil. Vaporizing some of the heavier fractions produces more valuable steam cracker feed. This can be accomplished by increasing the flash drum temperature to increase the cut. However, the resulting vaporized heavier fractions tend to partially condense in the overhead vapor phase resulting in fouling of the lines and vessels downstream of the flash/separation vessel overhead outlet.
Accordingly, it would be desirable to provide a process for converting materials in the liquid phase in the drum to materials suitable as non-fouling components for the vapor phase.